Radio doings (Dec 1930-Jun1932)

December, 1930 RADIO DOINGS Page Twenty-seven TUBE/ ACE VAEVE/ By K. G. ORMISTON When the English radio fan exclaims, "My jolly old valves have expired!" he really means that his tubes have gone blooey. We must admit that the Englishman's word for it is the more accurately descriptive, and that for once our so-expressive lingo fails. When Dr. Lee de Forest first put a filament, plate and grid into .1 glass bottle and pumped out the air, he named his brain child "Audion." Divers and sundry imitations soon sprang into being and their unlicensed bootleggers called them "Audiotrons," "Thermotrons," "Electron Relays" and what not. Most of them were tubular in shape, being about four inches long and an inch in diameter. The radio amateurs and experimenters of those days dubbed them "tubes" for short, and "tubes" they have remained, regardless of shape, size, color or creed. Dr. de Forest still insists they are "Audions" and a certain large manufacturer is equally insistent that such devices bearing their trade mark must be known as "Radiotrons," but through it all John Public swears that tubes are tubes, and sometimes at 'em. A movement was started to popularize the cognomen The elaborate and very complete tube testing equipmerit of Radw Manufacturers' Supply Company, 1000 South Broadway, Los Angeles "Bulbs," but the horticulturalists protested when it was found that a Cunningham, even if grounded, would not grow dahlias! But to get back to the English of it, the thing really is a valve — an electrical valve. It is a device wherein a very small force is made to control a greater amount of energy. A slight variation in voltage applied to the grid, results in a large change in the plate current. Hence comes the tube's ability to amplify. By means of several such tubes, connected consecutively, the original feeble impulse picked up by the antenna may be amplified millions of times, but the tube action is solely that of a valve, releasing more and more of the energy furnished by the power supply unit of the set toward the ultimate goal of producing audible sounds from the loud speaker. * * * The measure of a tube's merit obviously must be based upon the relation between the plate changes corresponding to grid changes. A figure which expresses this relationship is called the "amplification factor," which, together with the "plate resistance" determines the "mutual conductance" of the tube. These are the three vital factors. They have unfortunately remained somewhat of a mystery to the majority of radio service men who believe that they have completely tested a tube when they have measured its plate current! These important characteristics of a tube may readily be determined from readings taken with a plate milliammeter, a plate voltmeter and a grid voltmeter. Mutual conductance is in itself an excellent indication of the ability of a tube as an amplifier, since it takes into account the other two factors — plate resistance and amplification factor. Mutual conductance is expressed in mhos (conductance being the opposite of resistance, the unit of measurement is the ohm of resistance, spelled backwards) and is designated by the symbol "Gm." It is equal to the amplification factor (symbol "Mu") divided by the plate resistance (symbol "Rp") or Gm equals Mu divided by Rp. Thus a Cunningham C-327 tube has an amplification factor of 9, a plate resistance of 9000 ohms, and a mutual conductance of .001 mhos or 1000 micromhos (1 mho is 1,000,000 micromhos.) It is obvious from the above relationship that the most desirable condition exists when a tube has a high amplification factor and a low plate resistance as this will result in the greatest mutual conductance. A tube having a high Mu, but also a high Rp will show no increase in mutual conductance since one offsets the other. The mutual conductance of receiving tubes ranges from 400 to 2000 micromhos. The mutual inductance of a tube may be determined by the following method: Set the grid voltage at a value near the normal operating value, noting the voltage and calling it Eg-1. Set the plate voltage near the normal value, and then read the plate current resulting from these two adjustments, and call it Ip-1. Now increase the negative grid voltage and call this new value Eg-2. Read the plate current, calling it Ip-2. The plate voltage must remain constant during this test. Subtract the smaller plate current Ip-2 from the larger plate current Ip-1, calling the difference Ip-3. Then subtract the smaller grid voltage Eg-1 from the larger value Eg-2, calling the difference Eg3. Divide the difference in the plate current values Ip-3 (this must first be converted from milliamperes to amperes) by the difference in the grid voltage values Eg3, and the result will be the mutual conductance in mhos. To express in micromhos, multiply by 1,000,000. To illustrate as a formula: Mutual conductance (Gm) — Change in plate current Change in grid voltage For example: Assume that a grid voltage of —2.5 gives a plate current of cS milliamperes. Increasing the grid voltage to —7.5 causes the plate current to drop to 4 milliamperes. The plate current change is then .004 amperes, and the grid voltage change 5.0 volts. The above formula then gives the Gm as .000S mhos or 800 micromhos. Determining the Gm of any tube and comparing the value obtained with the manufacturer's rating affords an accurate indication of the condition of the tube. All service men and experimenters should have a copy of the Cunningham manual or engineering bulletins, for this purpose.